The processes for converting hydrocarbons at high temperature, such as for example, steam-cracking, catalytic cracking or deep catalytic cracking to produce relatively high yields of unsaturated hydrocarbons, such as, for example, ethylene, propylene, and the butenes are well known in the art. See, for example, Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055; Gartside et al., U.S. Pat. No. 4,814,067; Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465.
It is also well known in the art that these mono-olefinic compounds are extremely useful in the formation of a wide variety of petrochemicals. For example, these compounds can be used in the formation of polyethylene, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber.
Besides the mono-olefins contained in the cracked gases, the gases typically contain a large amount of other components such as diolefins, hydrogen, carbon monoxide and paraffins. It is highly desirable to separate the mono-olefins into relatively high purity streams of the individual mono-olefinic components. To this end a number of processes have been developed to make the necessary separations to achieve the high purity mono-olefinic components.
Plural stage rectification and cryogenic chilling trains have been disclosed in many publications. See, for example Perry's Chemical Engineering Handbook (5th Edition) and other treatises on distillation techniques. Recent commercial applications have employed technology utilizing dephlegmator-type rectification units in chilling trains and a reflux condenser means in demethanization of gas mixtures. Typical rectification units are described in Roberts, U.S. Pat. No. 2,582,068; Rowles et al., U.S. Pat. No. 4,002,042, Rowles et al., U.S. Pat. No. 4,270,940, Rowles et al., U.S. Pat. No. 4,519,825; Rowles et al., U.S. Pat. No. 4,732,598; and Gazzi, U.S. Pat. No. 4,657,571. Especially successful cryogenic operations are disclosed in McCue, Jr. et al., U.S. Pat. No. 4,900,347; McCue, Jr., U.S. Pat. No. 5,035,732; and McCue et al., U.S. Pat. No. 5,414,170.
In a typical conventional cryogenic separation process, as shown in FIG. 1, the cracked gas in a line 2 is compressed in a compressor 4. The compressed gas in a line 6 is then caustic washed in washer 8 and fed via a line 10 to dryer 12. The dried gas in a line 14 is then fed to the chilling train 16. Hydrogen and methane are separated from the cracked gas by partially liquefying the methane and liquefying the heavier components in the chilling train 16.
Hydrogen is removed from the chilling train 16 in a line 18 and methane is removed via a line 20, recompressed in compressor 24 and recovered in a line 26.
The liquids from the chilling train 16 are removed via a line 22 and fed to a demethanizer tower 28. The methane is removed from the top of the demethanizer tower 28 in a line 30, expanded in expander 32 and sent to the chilling train 16 as a refrigerant via a line 34. The C.sub.2 + components are removed from the bottom of the demethanizer tower 28 in a line 36 and fed to a deethanizer tower 38. The C.sub.2 components are removed from the top of the deethanizer tower 38 in a line 40 and passed to an acetylene hydrogenation reactor 42 for selective hydrogenation of acetylenes. The effluent from the reactor 42 is then fed via a line 44 to a C.sub.2 splitter 46 for separation of the ethylene, removed from the top of splitter 46 in a line 48, and ethane, removed from the bottom of splitter 46 in a line 50.
The C.sub.3 + components removed from the bottom of the deethanizer tower 38 in a line 52 are directed to a depropanizer tower 54. The C.sub.3 components are removed from the top of the depropanizer tower in a line 56 and fed to a C.sub.3 hydrogenation reactor 58 to selectively hydrogenate the methyl acetylene and propadiene. The effluent from reactor 58 in a line 60 is fed to a C.sub.3 splitter 62 wherein the propylene and propane are separated. The propylene is removed from the top of the C.sub.3 splitter in a line 64 and the propane is removed from the bottom of the C.sub.3 splitter in a line 66.
The C.sub.4 + components removed from the bottom of the depropanizer tower 54 in a line 68 are directed to a debutanizer 70 for separation into C.sub.4 components and C.sub.5 + gasoline. The C.sub.4 components are removed from the top of the debutanizer 70 in a line 72 and the C.sub.5 + gasoline is removed from the bottom of the debutanizer 70 in a line 74.
However, cryogenic separation systems of the prior art have suffered from various drawbacks. In conventional cryogenic recovery systems, the cracked gas is typically required to be compressed to about 450-600 psig, thereby requiring 4-6 stages of compression. Additionally, in conventional cryogenic recovery systems, four tower systems are required to separate the olefins from the paraffins: deethanizer, C.sub.2 splitter, depropanizer and C.sub.3 splitter. Because the separations of ethane from ethylene, and propane from propylene, involve close boiling compounds, the splitters generally require very high reflux ratios and a large number of trays, such as on the order of 100 to 250 trays each. The conventional cryogenic technology also requires multi-level cascaded propylene and ethylene refrigeration systems, as well as complicated methane turboexpanders and recompressors or a methane refrigeration system, adding to the cost and complexity of the conventional technology. It has also been studied in the prior art to employ metallic salt solutions, such as silver and copper salt solutions, to recover olefins, but none of the studied processes have been commercialized to date.
For example, early teachings regarding the use of copper salts included Uebele et al., U.S. Pat. No. 3,514,488 and Tyler et al., U.S. Pat. No. 3,776,972. Uebele et al. '488 taught the separation of olefinic hydrocarbons such as ethylene from mixtures of other materials using absorption on and desorption from a copper complex resulting from the reaction of (1) a copper(II) salt of a weak ligand such as copper(II) fluoroborate, (2) a carboxylic acid such as acetic acid and (3) a reducing agent such as metallic copper. Tyler et al. '972 taught the use of trialkyl phosphines to improve the stability of CuAlCl.sub.4 aromatic systems used in olefin complexing processes.
The use of silver salts was taught in Marcinkowsky et al., U.S. Pat. No. 4,174,353 wherein an aqueous silver salt stream was employed in a process for separating olefins from hydrocarbon gas streams. Likewise, Alter et al., U.S. Pat. No. 4,328,382 taught the use of a silver salt solution such as silver trifluoroacetate in an olefin absorption process.
More recently, Brown et al., U.S. Pat. No. 5,202,521 taught the selective absorption of C.sub.2 -C.sub.4 alkenes from C.sub.1 -C.sub.5 alkanes with a liquid extractant comprising dissolved copper(I) compounds such as Cu(I) hydrocarbonsulfonate in a one-column operation to produce an alkene-depleted overhead, an alkene-enriched side stream and an extractant rich bottoms.
Special note is also made of Davis et al., European Patent Application EP 0 699 468 which discloses a method and apparatus for the separation of an olefin from a fluid containing one or more olefins by contacting the fluid with an absorbing solution containing specified copper(I) complexes, which are formed in situ from copper(II) analogues and metallic copper.
However, none of the prior art absorption processes have described a useful method of obtaining relatively high purity olefin components from olefin-containing streams such as cracked gases. The use of silver nitrate solutions while good at separating olefins from non-olefinic hydrocarbon gases has generally proved to be impractical at separating the olefins from one another. Moreover, the hydrogen contained in the process stream has proven to be detrimental due to the chemical reduction of the silver ions to metallic silver in the presence of hydrogen.
Regarding the copper absorption processes, none of the processes disclosed to date have proven sufficient to provide the high olefin purities for the petrochemical industry, i.e., polymer grade ethylene and propylene.
In a recently filed patent application assigned to the same assignee as the present application, Ser. No. 08/696,578, attorney docket no. 696-246, a system especially suited for the use of cuprous salts with buffering ligand (although silver salts and other metallic salts were also disclosed in connection therewith) was disclosed. Although the cuprous salt system provided several advantages over the prior art, the use of a system especially suitable for employing silver ions has certain further advantages. For example, unlike silver+1! ions, cuprous ions are not stable and require a buffering ligand. Accordingly, various systems are required for preparing the buffered cuprous salt solution and for containing and recovering the ligand. Additionally, cuprous salts are not as soluble as silver salts, such as silver nitrate, thereby requiring a greater solution circulation rate and larger equipment. Although silver nitrate is considerably more expensive than its copper counterparts, it is contained in the system and can readily be recovered from spent solution.
Therefore, it would be highly desirable to provide a economical system which is especially suitable for the use of silver salts as the chemical absorbent.